Vinyl halide resins are widely employed in the plastics industry as fabricating and packaging materials, coatings, and a variety of other applications. In these applications and in processing, vinyl halide resins must be stabilized aginst degradation by heat and light. Otherwise, when vinyl halide resins are processed or molded under elevated temperatures for extended periods, they tend to degradate. Degradation greatly reduces the mechanical properties of the resins and destroys their clarity and color. Many stabilizers have been proposed to reduce or largely prevent the degradation of vinyl halide resins under processing and service conditions. A somewhat distinct industry in commerce, most commonly called the "stabilizer" industry, has developed over the years whose principal purpose has been to provide better stabilizers for vinyl halide resin compositions. A very large amount of patent literature and other publications have been addressed to the development of vinyl resin heat and light stabilizers. Most commonly, these stabilizers have been based upon various metallic soaps, phenates, phosphites and other salts.
Early in the development of vinyl halide resin stabilizers, zinc salts or soaps were suggested. Since then, a significant amount of effort has gone into the development of stabilizers based upon zinc principally because of their cheapness, non-toxicity and prevention of atmospheric staining by sulfur compounds. It is well-known, however, that zinc soaps alone are not good primary stabilizers because zinc causes a phenomenon referred to in the art as "catastrophic degradation" of the vinyl halide resin. This phenomenon occurs when vinyl halide resins are processed at elevated temperatures. Initially, the resin has excellent color and clarity for a period of processing time, but then suddenly turns black wtihin a few seconds. More recent patents which refer to this phenomenon of catastrophic degradation include U.S. Pat. Nos. 3,262,896 and 3,575,905. More detailed theoretical discussion can be found in the book "The Stabilization of Polyvinyl Chloride" by F. Chevassus et al. (St. Martin's Press, Inc., New York, 1963).
Principally because zinc salts are cheap and non-toxic among other advantages, the stabilizer industry has actively pursued ways to solve the catastrophic degradation phenomenon. Among the various approaches proposed to overcome the deficiency of zinc include the addition of alkaline earth or alkali metal salts to the zinc stabilizer. A number of years ago, for example, it was reported that the sudden blackening of resins stabilized with zinc stearate could be delayed by employment of alkali metals or alkaline earth metals such as barium. However, in commercial operation, it was soon found that the zinc-barium combination was, in most cases, not a satisfactory solution to the problem because the barium component had to be used in relatively large amounts. Also, such large amounts of barium imparted to the resin what chemists have called "early color" which is to be avoided in clear resin stock applications. More recently, for example, in U.S. Pat. No. 3,262,896, potassium soaps were proposed for usage with zinc soaps with better results reported than previous combinations of barium or sodium soaps with zinc soaps. In addition, many other compounds have been proposed for use in combination with zinc with reportedly desirable results such as those mentioned in U.S. Pat. No. 3,067,166 where zinc compounds in combination with mercapto acid/primary alcohol esters were employed. In substance, the use of either alkali metal salts, alkaline earth metal salts, organic phosphites, hindered phenols, epoxidized oils and the like, will delay somewhat the phenomenon of sudden degradation, but such use has not been completely satisfactory. The divalent zinc component of these stabilizer systems can still only be used in minor amounts to avoid catastrophic degradation and, therefore, cost and performance advantages associated with using larger amounts of zinc have simply not been achieved.
In addition to the attempts which have been made to extend the heat stability of zinc stabilizers by the employment of other stabilizer ingredients, efforts have been made to define preferred ranges of zinc in the presence of other stabilizers so as to achieve the optimum concentration of zinc and still avoid sudden degradation. As previously reported in U.S. Pat. No. 3,575,905, it was found that small amounts of zinc, when combined with an aryl alkyl phosphite, are effective in deterring thermal degradation of vinyl halide resins. As disclosed in this patent, however, when zinc is present in concentrations higher than about 0.005 part of divalent zinc per 100 parts of resin, rapid degradation will result and this degradation is not prevented by any known auxiliary additives.
Metal organophosphorus compounds have been suggested in U.S. Pat. Nos. 3,467,683 and 3,284,386 where a divalent metal is linked through an oxygen to a phosphorus atom of an aryl phosphite and a carboxylic acid radical is attached to the other valence of the divalent metal. These compounds may be termed "half-soap" reaction products.